I have a question, I started to learn Sanskrit, and I thought a little about it, as I am going to be a monk, I suppose I need to learn Pali, don't I?So, I have a few questions:

1. If I know Sanskrit well, will I be able to understand Pali as well?

Sanskrit is a great deal more formally constructed (pun intended) than is Pali, which is a prakrit, a Sanskrit relative. It is easier going from Sanskrit to Pali than Pali to Sanskrit. If you are good with languages, learning Sanskrit first would be the way to go, though learning Sanskrit is not necessary to learning Pali.

2. Do monks learn Pali at the monkshood with their Ven. Master or teacher? Or they need to learn it before? I ask because if not, I guess I'll stop learning Sanskrit and move on to Pali.

Monks, for the most part, learn Pali chants, but not necessarily the language itself, unless the monk is going a scholarly route.

3. Is there any word list? Like most used Pali words? Like 10,000 words? Or "List of words, which with them you can read most Suttas" etc....I prefer to have a list than translating one one.

Like any language, it more than just vocabulary. Pali is a highly inflected language, which means that meaning of words are determined by the usage and context of the word. And that means a mastery of Pali grammar is an absolute must in understanding how a word is used and what is being said by the usage of the word in question, and in that a strong knowledge of Sanskrit can be a big help. For studying Pali, A.K. Warder's INTRODUCTION TO PALI is a good place to start.

This being is bound to samsara, kamma is his means for going beyond. -- SN I, 38.

“Of course it is happening inside your head, Harry, but why on earth should that mean that it is not real?” HPatDH p.723

>> Do you see a man wise[enlightened/ariya]in his own eyes? There is more hope for a fool than for him.<<-- Proverbs 26:12

If your main interest is Theravada, Pali is clearly more useful than Sanskrit. Therefore my recommendation to a beginner would be to start with Pali. On the other hand, there are a few grammatical details in Pali that are easier to understand by comparing them to Sanskrit. So Sanskrit is also useful, both for deeper understanding of Pali and for giving you the ability to read Buddhist texts in Sanskrit from other early schools. And once you know one of these two languages, it is fairly easy to learn the other one.

Since you have started learning Sanskrit, I will recommend that you continue and finish the course. When you start learning Pali afterwards, you will find that there is a lot of common ground, so learning Sanskrit is not a waste of time.

You ask for a word list. There exists a book with the common top 1000 words of the Pali suttas, which may be useful to you: Kurt Schmidt, "A Frequency Dictionary of Pali", Theravada Tipitaka Press, 2010, ISBN 978-1453870136. I don't find it any longer at the publisher's website, but you can of course mail them and ask if there might be some forgotten copy on a shelf somewhere.

I speak many languages, including Chinese, Hebrew, Polish, German and so on.I am planning to move to Thailand in 3-5 years from today, so I guess it's more like to learn Sanskrit OR Pali, I like how Sanskrit is written and sounds - so many words with a funny pronunciation, and I really want to read ancient books in it, such as Mahabharta, Upanishads etc.By the way, I started learning Sanskrit only a few days ago, I can read Devanagari fast, since I learned to read the alphabet 3 years ago because it was different and unique, so now I am just learning words, I know some around ~100, and a little grammar (like a few rules in Sandhi, and some verbs conjunctions, nothing big), it's not like I am 2nd year at the University so I wanted to ask before I deeply continue to learn Sanskrit, perhaps it is just a waste of time and it'll be better to master Pali, and if I'll have time, I will learn Pali, even basic, so?

But on the contrary, it would be such a great idea to learn Pali, since I am about to join a monkshood, although they will speak Thai there, and since it's Theravada Thai Forest Tradition, no chanting, we meditate only with listening to our own breathe, while not controlling it, simply listening, with its own speed and rate.

Learn them both at the same time? Right now I am kind busy, and I don't want to add another 5th language to my learning schedule, although it's a good and important one in my case, what do you think?

You must decide for yourself. But as for me, I would not recommend studying Pali and Sanskrit at the same time. That is to invite confusion. Better get a solid grounding in one of them before you dive into the other one. So do you want to start with Sanskrit or Pali? The choice is yours, but I wish you the best of luck whatever choice you make.